As a client application using MassTransit, I want to make requests to a service and receive an asynchronous wait object in response that I can use to optimize my request processing so that I don't tie up the .NET thread pool with pending requests.

1. When submitting a request message, the service bus should return an IAsyncResult that can be used to wait for the response.

2. When the response arrives at the service bus, the service bus should set the wait handle to release the requesting thread.

3. When the response arrives at the service bus, if a callback was registered with the request, it should be invoked by the service bus.

1 and 2 are done. the callback support needs to be added yet.

Test for callback written and functionality verified.